1. Field of the Invention
The present invention relates to an image processing apparatus, as well as to an image processing method, and more particularly to image processing apparatus and method which can effectively suppress degradation of image quality at an image region which contains a predetermined color.
2. Description of the Related Art
ISO/IEC DIS (Draft International Standard) 10918-1 and ITu-T recommendation T.81 have been determined as standard methods for encoding and decoding still picture. These standards are the same in the contents and are collectively referred to as "JPEG Standards" in accordance with the practice in the field concerned.
According to JPEG standards, color image signals are subjected to discrete cosine transform (referred to as "DCT", hereinafter) on block basis, and the transform coefficients of DCT are quantized with variable stepsize so as to encode the quantized data into codes of variable lengths, thus achieving compression of the amounts of the codes. In this process, the quantization stepsize is controlled in accordance with one of a plurality of predetermined quantization tables. The variable-length codes used in the encoding are determined in accordance with a pair of encoding tables which are selected from groups of encoding tables given for D.C. and A.C. components of the DCT coefficients.
It is to be noted, however, JPEG standards do not pose any restriction in regard to the reference for the selection of quantization and encoding tables and the manners in which the contents of both types of tables are determined.
This known technique has suffered from the following problem. The compression ratio in the image encoding method following JPEG standards or similar to the same relies upon the reference for the selection of the quantization table and the pair of encoding tables and upon the content of each quantization table and each encoding table. In other words, the compression ratio is ruled by the manner of determination of the quantization stepsizes and the variable-length codes.
Variable-length codes optimum for improving the compression ratio can be determined by examining the statistical distribution of the quantized data obtained through quantization of the DCT coefficients. By optimizing the variable-length codes, it is possible to improve to a certain degree the compression ratio without being accompanied by degradation of the quality of the reconstructed image. This method, however, cannot provide a remarkable increase in the compression ratio.
Obviously, the greater the quantization stepsizes, the greater the compression ratio. Increases in the quantization stepsizes, however, cause a degradation in the quality of the reconstructed image. It is therefore necessary that the quantization stepsizes are determined so as to achieve the required compression ratio without causing any noticeable degradation of the image quality.
ITu-T recommendation H. 261 has been enacted to provide standards for methods of encoding and decoding moving picture components for audio and video services such as those for videophone, video cnnferencing system, or the like. According to ITu-T recommendation H. 261, encoding at a very high compression ratio is realized thanks to the use of variable quantization stepsize and the use of variable-length coding. In addition, a constant transmission rate is attained by the use of a transmission buffer.
In order to realize a constant transmission rate, it has been proposed to feedback the amount of data in the transmission buffer as control information, so as to vary the quantization stepsize in accordance with the amount in the transmission buffer. According to this method, when there is a risk of overflow of the transmission buffer due to an increase in the amount of data, the quantization stepsize is increased so as to suppress generation of transmitted data, whereas, when there is a risk of under-flow of the buffer due to decrease in the amount of data, quantization stepsize is reduced so as to promote generation of the data to be transmitted.
In general, however, in the known moving picture signal encoding apparatus of the type described, the quantization stepsize is set to comparatively large value as an average when the transmission rate is low, so that defect or degradation of image such as block distortion tends to be caused in the reconstructed image.